ARM: imx27-dt: Fix build due to removal of irq_domain_add_simple()

commit 6b783f7c (irq_domain: Remove irq_domain_add_simple()
replaced irq_domain_add_simple with irq_domain_add_legacy()

Implement this conversion so that imx27-dt can be built again.

Reported-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>